The present invention relates to an electronic ballast circuit and a method of operating the circuit. The electronic circuit detects and limits peak current surge, specifically peak current surge through a high intensity discharge (HID) lamp.
An inrush of current occurs after ignition of a typical D1 HID lamp. This current nominally has a peak of 30 A. A lamp of this type, like any lamp used in automotive HID circuitry, uses an H-bridge, or Full Bridge, as a switching circuit to provide AC current to the lamp. The 30 A surge can cause unreliable operation of the H-bridge control circuitry, or failure of the H-bridge MOSFET switches.
Handling high current through an electronic ballast circuitry can be accomplished in a variety of ways. One way is to utilize components, such as MOSFETs, that are rated at a given current level that is as high or higher than the maximum current through the system. One drawback of using components having a high current rating is that they are more expensive. Also, the higher rated components tend to be larger and heavier than their lower rated counterparts. In most applications, the space occupied by the components and the weight of the components should be kept to a minimum.
A second way of handling the excess current is to install a component that provides for controlled inrush of xe2x80x9cONxe2x80x9d current. Such components include, but are not limited to, a thyristor or pair of thyristors in an active filter-voltage step-up circuit as disclosed in U.S. Pat. No. 5,719,473 to Huber et al; an output stage designed as a step-up regulator or a blocking oscillator as disclosed in U.S. Pat. No. 5,877,614 to Huber; an oscillatory transformer as disclosed in U.S. Pat. No. 6,078,144 to Twardzik; a multivibrator with hysteresis as disclosed in EP0 757,420; or an inductor of sufficient volt-second capability. The inductor would normally be placed in series with the lamp to limit the peak current. Installing any of these components increases the overall size, complexity and cost of the system.
It is an object of the present invention to utilize an existing lower rated MOSFET in an H-bridge circuit to limit current flowing therethrough.
Another object of the present invention is to reduce the gate voltage of a MOSFET in an H-bridge during the period following lamp ignition to limit current flowing through the MOSFET.
A further object of the present invention is to provide a method for limiting the current through the MOSFET using parasitic capacitance.
The present invention achieves these and other objects by using at least one of the existing H-bridge MOSFET switches to limit the D1 lamp surge current to a level that is within the rating of the existing devices. The present invention makes use of a parasitic capacitance that is created by operating the circuit in the preferred embodiment. Drawing voltage from one of the resistors in the circuit in order to discharge the parasitic capacitance reduces the voltage at the gate of the MOSFET. The gate then partially closes and the current across the MOSFET is then limited. An advantage of using this approach is that by detecting and limiting the surge, the addition of external components or costly oversized switching MOSFETs can be avoided.
This invention may be clearly understood by reference to the attached drawing which shows an electronic ballast circuit for an HID lamp according to a preferred embodiment of the invention.